1. Field of the Invention
The present invention generally relates to heat exchangers, such as those of the type used in air-conditioning systems. More particularly, this invention relates to a manifold configuration that is resistant to deformation during a piercing operation to form holes or slots in which cooling tubes can be inserted.
2. Description of the Related Art
Heat exchangers are employed within the automotive industry as condensers and evaporators for use in air conditioning systems, radiators for cooling engine coolant, and heater cores for internal climate control. In order to efficiently maximize the amount of surface area available for transferring heat between the environment and a fluid flowing through the heat exchanger, heat exchanger designs are typically of a tube-and-fin type in which numerous tubes thermally communicate with high surface area fins. The fins enhance the ability of the heat exchanger to transfer heat from the fluid to the environment, or vice versa. For example, heat exchangers used in the automotive industry as air conditioner condensers serve to condense a vaporized refrigerant by transferring heat from the refrigerant to the air forced over the external surfaces of the condenser.
One type of heat exchanger used in the automotive industry is constructed of a number of parallel tubes that are joined to and between a pair of manifolds, creating a parallel flow arrangement. Internal passages within the manifolds define reservoirs that are in fluidic communication with the tubes through tube ports, e.g., holes or slots, formed in the manifolds. One or both manifolds include one or more inlet and outlet ports through which a coolant enters and exits the heat exchanger. Conventionally, such heat exchangers have been constructed by soldering or brazing the tubes to their respective ports. Finally, fins are provided in the form of panels having apertures through which the tubes are inserted, or in the form of centers that can be positioned between adjacent pairs of oblong or flat tubes.
The process by which tube ports are formed has often entailed a significant number of processing steps in order to accurately shape the ports, such that minimal material is employed to achieve a sufficiently strong joint for the intended application. One type of tube port known in the prior art consists essentially of an opening in the manifold wall, as shown in U.S. Pat. No. 5,622,220 to Park et al. While forming such openings generally involves a single punching operation, a drawback of this port configuration is the minimal amount of material available to engage and bond with a tube assembled with the port. In addition, a mandrel or other tooling must typically be used to support the inner diameter of the manifold, and the slug produced by the punching operation must be collected and discarded. These requirements undesirably add complexity and cost to the forming equipment.
A second type of tube port configuration employed in the prior art includes a riser or collar that provides a substantially greater amount of material for engagement with the tube. Risers that protrude outward from a manifold are more difficult to form than a simple opening in a manifold, and have conventionally entailed multiple forming operations. An example of an improved process for forming risers is disclosed in commonly-assigned U.S. Pat. No. 4,663,812 to Clausen. Clausen""s process involves forming a projection on a manifold that is then further formed or machined to create a solid riser, which subsequently undergoes a reverse impact extrusion process to form a tubular riser. Collars that protrude into the internal passage of a manifold can be formed by piercing operations that are typically less complicated than processes required to form risers. However, piercing operations can collapse a manifold unless the internal passage of the manifold is adequately supported with a mandrel or other suitable tooling. Another complication encountered when attempting to pierce a tube port in a manifold is the tendency for the manifold to rotate as a result of any asymmetric forces applied by the piercing operation, necessitating the use of appropriate fixturing to secure the manifold.
In view of the above, there is a desire to simplify the process for forming a tube port in heat exchanger manifold. Such an improved process would preferably avoid collapsing of the manifold and minimize the number of steps necessary to form the port, yet consistently yield a port that promotes the joint strength of the tube-port assembly.
The present invention provides a method for forming tube ports on a heat exchanger manifold, and a manifold for use in such a method. The invention is particularly suitable for manifolds having an internal passage and an outer cross-sectional shape that are substantially uniform along a length of the manifold, such that the manifold can be formed by extrusion or seam welding.
According to the invention, the manifold comprises first and second walls that define the internal passage and the outer cross-sectional shape of the manifold. The first wall of the manifold has a concave outer surface in which the tube ports of the manifold are formed, while the second wall has a convex outer surface. The concave and convex outer surfaces of the first and second walls, respectively, cause the outer cross-sectional shape of the manifold to be crescent-shaped. The method of this invention generally includes forming the manifold to have the first and second walls as described above, and then forming the tube ports in the first wall, preferably by piercing. According to the invention, a manifold having a crescent-shaped cross-sectional shape is able to resist deformation while a piercing, punching, or other hole-forming operation is performed on the concave surface of the first wall, even if the internal passage of the manifold is not supported. Instead, piercing and other hole-forming operations performed on the concave surface result in moderate stretching of the first wall in the vicinity of the location where piercing takes place.
From the above, it can be seen that the cross-sectional shape of manifolds formed in accordance with this invention allows for tube ports to be formed by a simplified process that requires a minimal number of processing steps, substantially all of which can be performed while the manifold is supported within a single die cavity or other suitable fixturing. An additional advantage of the crescent-shaped manifold of this invention is that the opposite extremities of the manifold provide locations at which the manifold can be clamped to prevent rotation of the manifold in the die cavity, leading to improved dimensional uniformity and consistency of tube ports on a manifold. If a piercing operation is used, tube ports formed by the method of this invention may include a collar that increases the amount of material available to engage and bond to a heat exchanger tube, thereby promoting the joint strength between the port and the tube. Another advantage of the invention is that the configuration of the manifold allows for the use of piercing processes in a wider range of product designs.